1. Field of the Invention
The invention concerns lasing systems and, more specifically, a system in which the Q of a resonant cavity is spoiled by the use of the stimulated Raman effect, without using a conventional shutter.
2. Discussion
As shown in FIG. 1, a typical laser includes a lasing medium 3 positioned between two mirrors 6 and 9. The spacing 12 between the mirrors is an integral number of half wave lengths of the laser frequency. That is, distance 12 equals (N) (lambda)/2, where N is an integer and lambda is the wave length. Therefore, mirrors 6 and 9 form a cavity which can support standing waves at the wave length lambda.
Q is a figure of merit which refers to the sharpness of the resonance in the cavity. A technique called Q-switching is frequently used to obtain short, intense bursts of light from the laser cavity.
In Q-switching, a shutter 15 closes and obstructs one of the mirrors, mirror 9 in this case, thereby preventing photons from travelling back and forth between the mirrors. The closed shutter prevents oscillations by reducing (or spoiling) the Q of the cavity. Without oscillation, stimulated emission is inhibited, and increasing population inversion in the lasing medium 3 is promoted. When the shutter 15 is opened, the cavity becomes high-Q, and reflections between mirrors 6 and 9 resume, thereby stimulating emission and allowing a depletion of the population inversion. The depletion is rapid, resulting in a brief, intense pulse of light.
Several types of shutter can be used. Electro-optic, magneto-optic, or acousto-optic modulators can be used, as well as a bleachable dye which becomes transparent in the presence of sufficient photons. Also, a rotating mirror can be used. However, all of these shutters, with the exception of the dye switch, tend to be expensive. As to a dye switch, some environmental conditions can prohibit the use of dye Q-switches.
Researchers have used Raman cells with Q-switching to generate brief light pulses. For example, R. Frey, A. deMartino and F. Pradere, in "High-Efficiency Pulse Compression With Intra-Cavity Raman Oscillators," Optics Letters, Volume 8, Number 8, Page 437, August, 1983, discuss the use of intra-cavity Raman cells to generate short pulses at the first Stokes frequency. In addition, F. deRougemont, Ding Kong Xian, R. Frey, and F. Pradere, in "High Efficiency Pulse Compression With Externally Pumped Intra-Cavity Raman Oscillators", Optics Letters, Volume 9, Number 10, Page 460, October, 1984, discuss the use of a high Q resonator at the first Stokes frequency used to generate short pulses at the second Stokes frequency. However, these researchers all use a conventional Q-switch in their apparatus.